The present invention relates to a continuous caster suitable for use in a hot rolling mill system in which steps from continuous casting to finish rolling are performed in a direct rolling manner, and more particularly to a thin slab continuous casting machine and method for continuously casting a thin slab with a thickness not greater than 100 mm.
About thirty years has passed since continuous casting was put to practical use, and most of slabs is now produced by the continuous casting. Heretofore, in consideration of quality, slabs with a thickness ranging from 200 mm to 250 mm (hereinafter referred to as thick slabs) have generally been produced by the continuous casting at a casting speed of 1.5 to 2.5 m/min (hereinafter referred to as first prior art).
Meanwhile, in the 1980's, a hot rolling mill system called a continuous casting--hot rolling--through line (or direct-feed) system, in which steps from continuous casting to finish rolling are performed through one line in a direct rolling manner, has been developed. As a result, taking into account the total production efficiency, a thin slab continuous caster capable of producing slabs with a thickness ranging from 30 mm to 100 mm (hereinafter referred to as thin slabs) is developed.
A hot rolling mill system (hereinafter referred to as second prior art) described in "Ein Jahar Betriebserfahrung mir der CSP-Anlage fuer Warumbreitband bei Nucor Steel"; Stahl u. Eisen, 111 (1991) Nr. 1, for example, is basically arranged such that a slab is directly fed from a continuous caster to a roughing mill. Exactly speaking, however, a slab is not continuously fed from the continuous caster to the roughing mill, but after being cut off into pieces with a length of 20 m to 50 m. Thus, since a slab is cut off into pieces between the continuous caster and the roughing mill, the casting speed and the rolling speed can be set independently of each other, meaning that the rolling speed can be increased regardless of the casting speed. Further, this prior art is premised on using steel molten by an electronic furnace (hereinafter referred to as electronic furnace steel) and can perform casting at a substantially constant speed under control of the amount of molten steel supplied.
Also, in a hot rolling mill system (hereinafter referred to as third prior art) described in "ISP-Thin slab challenge to Nucor"; Steel Times, Oct. 1993, a slab is not cut off between a continuous caster and a roughing mill, but continuously fed therebetween. In this system, however, a strip is once reeled up into a coil between the roughing mill and a finishing mill. Since the strip is unreeled from the coil in which the strip temperature is kept from lowering and then fed to the finishing mill, the rolling speed in the finishing mill can also be increased regardless of the casting speed. Further, as with the above second prior art, since this prior art is premised on using electronic furnace steel, it is possible to easily adjust the amount of molten steel supplied and to hold the casting speed substantially constant without considerable variations.
On the other hand, as disclosed in JP, A, 62-64462, there is known a technique for cooling and solidifying a slab in continuous casting with an arrangement that reheating and controlled cooling are selectively switched over in a cooling zone below a mold (hereinafter referred to as fourth prior art). More specifically, controlled cooling means is disposed in part of a non-solidified reheating zone, which is provided as a cooling region, to thereby establish a zone where controlled cooling can be performed. When the casting speed is fast, the controlled cooling means is actuated under control for cooling a cast slab, and when the casting speed is slow, only the nonsolidified reheating is effected without carrying out the controlled cooling. Thus, the cooling rate is controlled depending on the casting speed so that the temperature of a cast slab is kept at a desired value. Also, it is considered to adjust the cooling rate so that the crater end of a cast slab reaches substantially the same position as the end of a continuous caster or enters the caster.